Process for the recovery of cryolite from waste gases and from the carbon bottom of an aluminum electrolytic furnace



Nov. 20, 1962 H. MADER 3 065 051 PROCESS FOR THE REOOMERT7 OF CRYOLITE FROM WASTE GAsEs ANO FROM TRE CARBON BOTTOM OE AN ALUMINUM ELECTROLYTIC FURNACE Filed Aug. 24, 1960 waste. Gas .Free Ground From Fluorme. Comp. Carbon-bonen] l -2I7 pmol Lu: ausm Residue d Sodium Carbonute- A 4f t Sodo Lye Lyeglufon gl 0 or d P 20L n J T l xmlsl" k/j l l' l' I l '.'l le T alf I 21E p pHconrol ggg @.99 al) 2.2@ w h L. .d

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INVENTOR.- Herbert Mader AGENT.

PRQCESS EGR THE RECVERY F CRYOLETE FRUM WASTE GASES AND BRGM THE CARBON BTTOM 0E AN ALUMINUM ELECTRLYTEQ EURNAGCE Herbert Mader, Upper Austria, Austria, assigner to Vereinigte Metallwerke Ranshofen-Berndort Aktiengesellschait, Upper Austria, Austria, a corporation of Upper Austria Filed Aug. 24, 1960, Ser. No. 51,687 9 Claims. (Cl. 23-88) The instant invention is directed to a novel process for recovering cryolite from Waste gases and from the carbon bottom of an aluminum electrolytic furnace.

The losses of fluorine which occur during the fusion electrolysis of aluminum derive, as is known, from the fact that on the one hand some cryolite is decomposed both electrolytically and thermally and volatizes in the form of gaseous fluorine compounds, and on the other hand porous carbon bricks of the furnace bottom absorb considerable amounts of fused cryolite which, after the furnace'bottom is exhausted, is cleared away and is lost. In order to avoid losses arising this way, diierent recovery processes have been developed during the course of time and have been used in the electrolytic aluminum practice.

The processes for recovering fluorine from the waste gases of aluminum electrolytic furnaces depend essentially on the washing of the gases with dilute sodium carbonate solution and on the conversion of the sodium fluoride resulting in accordance with the equation into cryolite (Na-BAlFS) by any of the known methods. The sodium carbonate solution is circulated in the absorption equipment until it becomes converted, as per Equation l, into sodium fluoride so that, on the one hand, the alkalinity of the sodium carbonate solution is exploited as far as possible and, on the other hand, a sodium fluoride solution of relatively high concentration (30 to 35 grams per liter) is obtained.

The cryolite contained in the carbon bottom, however, is subjected in a crushed condition to treatment with dilute caustic soda lye whereupon sodium fluoride and a sodium aluminate liquor are obtained. The sodium fluoride and the sodium aluminate liquor are mixed with the necessary amount of sodium fluoride solution from the waste-gas washing. According to current knowledge a surplus of about 30% lluoride ions in relation to the aluminum ions present is required for the precipitation of cryolite. From the resulting mixture cryolite is precipitated by raes atet neutralizing the free alkali. The precipitation is accomplished, as is known, by the addition of carbon dioxide or sodium bicarbonate to said mixture, whereby cryolite is produced in accordance with one of the lfollowing equations:

3,065,051 Patented Nov. 20,4 l 962 lee NazCOs-l-HF-)NaF-l-NaHCOa (4) The reaction as per Equation 4 is only the rst stage of the customary neutralization process illustrated by Equation 1. The second stage, eliminated in the process according to the present invention, corresponds to the reaction equation l have found that it is possible to halt the neutralization process at the iirst stage of the reaction by maintaining the pH of the washing solution between substantially 9 and 12. The result is an equimolar solution of sodium iluoride and sodium bicarbonate, substantially all of the latter deriving from the reaction between the sodium carbonate and the gas.

A further advantage is the inherent speed of the process according to the invention since the reaction of Equation 4 is faster and more complete than that of Equation 5, and consequently more rapid and thorough than the reaction shown in Equation 1. This accelerated and thorough process of waste-gas Washing and cryolite recovery considerably improves the eiciencyl of absorbing the hydrogen uoride and, therefore, the yield of sodium uoride from the absorption equipment as compared with the previously used processes.

In addition to the reaction according to Equation 4, which is essential for the recovery of fluorine compounds, the sodium carbonate reacts also with the carbon dioxide and sulfur dioxide contained in the waste gases during the waste-gas washing according to the following equations:

The carbon bottom of an aluminum electrolytic furnace is comminuted by grinding. To this ground carbon bottom, containing cryolite, is added an excess (relative to the cryolite content) of dilute caustic soda lye. The insoluble matter is tiltered off from the obtained aluminate liquor which then contains sodium fluoride.

The solution of sodium fluoride and sodium bicarbonate obtained by the reaction illustrated by Equation 4 is purified by the removal of `suspended matter and is combined with the aluminate liquor obtained from the aforementioned treatment of the ground carbon bottom. Cryolite is precipitated, according to the reaction of Equation 3, and separated from the mother liquor, which contains soda, this mother liquor being then reused for neutralizing hydrogen fluoride which is contained in the waste gases.

The mother liquor resulting from the reaction illustrated by Equation 3 contains about 40 g./l. (grams per liter) or Na2CO3 and about 5 g./l. of NaF. (An excess of sodium uoride was used for the cryolite precipitation.) This mother liquor is again subjected, in conventional gas-purifying apparatus, to the action of Waste gases (containing hydrogen uoride, SO2 and CO2) for a time just sufficient to convert the Na2CO3 to NaHCO3, Nal? and Na2SO4. Furthermore, during the washing of the gas, solid and liquid water-insoluble suspended substances, e.g. alumina and cryolite dust, soot and tar, are are retained. These substances collect in the form of a la suspension in the washing solution. In order to separate them, the washing solution, which carries these suspended substances, is circulated via a settling basin or a concentrator. The washing solution which is withdrawn from the circulation of the fluid of the gas-purifying apparatus and used for the precipitation of the cryolite contains about 32 g./l. sodium bicarbonate and 15 g./l. sodium uoride. Only g./l. of the latter have been newly added by the passage through the gas-purifying apparatus. The additional use of soda, arising on account of the incomplete utilization of the entire soda alkalinity, is compensated by the addition of sodium hydroxide from the lixiviation of the bottom carbon and by the free-alkali content of the lixiviated bottom carbon.

It has been found advantageous for the preparation of aluminate liquor, containing sodium fluoride, to treat crushed (or ground) carbon bottom with sodium hydroxide in an amount exceeding by about 30% the quantity theoretically necessary. A solution is prepared in another apparatus from ground bottom carbon and caustic soda lye which contains per liter about 26 grams sodium iluoride and 3 to 4 grams aluminum in the form of aluminate as well as the surplus sodium hydroxide used for the lixiviation which is increased by the free alkali contained in the lixiviated bottom carbon.

It is desirable to grind the carbon bottom and treat it with lye immediately after clearing away the furnace bottom in order to recover the free alkali which might be contained in this material, since a loss in the free-alkali content occurs during storage. This loss is brought about by contact with precipitation water (washing out) and by the effects of atmospheric carbon dioxide.

In a further embodiment of the instant invention, after lixiviating the carbon bottom with an excess of dilute caustic soda lye and separating the residue from the liquor (aluminate liquor which contains sodium uoride), additional aluminate is prepared by adding aluminum, e.g. in the forms of turnings, foundry dross or aluminum compounds which are soluble in sodium hydroxide, to said liquor. Since the liquor contains an excess of sodium hydroxide, a reaction proceeds as follows:

2Al+2NaOH+H2O 2NaAlO2+3H2T (8) v The sodium-uoride-containing aluminate liquor, which besides 26 grams NaF/l. contains now 7 to 8 grams Al/l., is suitably added in such an amount to the sodium fluoridecontaining sodium bicarbonate solution withdrawn from the gas-purifying apparatus that, after reaction, the mother liquor separated from the cryolite, having a pH of at most 12, still contains 10 to 30% of the obtained amount of sodium uoride dissolved in the form of Na3AlF6. At such a mixing ratio the amount of sodium bicarbonate in the sodium uoride-containing bicarbonate solution is in any case large enough to reduce the alkalinity of the solution mixture to the degree of alkalinity of the soda (pH about 12) and thereby to precipitate the cryolite.

The above-described process, as disclosed in my copending application Ser. No. 759,154 led September 5, 1958, now abandoned, of which the present application is a continuation-in-part, achieves its greatest economic importance when it is carried out continuously. In such a continuous working process the four reaction systems are coupled and synchronized with one another. These four systems are:

Na3AlF6-l-4NaOH- 6NaF-{NaAlO2|2H2O (9) (III) The enrichment of the aluminate content of the liquor produced by the reaction represented by Equation 9 by reacting aluminum with the excess sodium hydroxide in said liquor, according to Equation 8; and

i (IV) The precipitation of cryolite by reacting together the products of the reactions represented by Equations 4 and 8, 9, after the solids have been removed from these products, in accordance with Equation 3.

After the cryolite is separated from the product of the reaction represented by Equation 3, the remaining sodium carbonate liquor may be used, according to the reaction of Equation 4, to neutralize hydrogen fluoride in waste gases.

According to this invention the continuous performance of the four reaction systems noted above may be carried out in such a way that the bicarbonate solution (containing sodium uoride) is separated from the suspended matter contained therein; the aluminate liquor (containing sodium fluoride) is filtered from the insoluble matter therein; the thus purified bicarbonate solution and aluminate liquor are continuously mixed; and the precipitated cryolite is thereafter removed from its mother liquor. The last-mentioned step may be accomplished by ltration.

Thus, the process consists of four partial processes which are logically interconnected, as described in greater detail below.

I. Waste-gas washing according to the equations:

The amount of Na2C03 consumed according to the Equation 7, is to be attributed to the sulfur content of the anode mass and is a complete loss. The sodium `sulfate is absorbed partially by the recovered cryolite. The remainder enriches the solutions which are present in the circulation and separates out as a precipitant at the bottom of the liquid when the limit of solubility is surpassed. As such it can be separated from the process solutions.

II. The leaching of the cryolite and of the aluminum compounds, both of which are soluble in NaOH, or of the metallic aluminum from the clearing of the carbon bottom by dilute caustic soda lye which is contained in the residue newly removed from the carbon bottom, and if necessary by additional free alkali, is accomplished according to the following reactions:

III. The enrichment of the aluminum content of the carbon-bottom lye by dissolving metallic aluminum, in the form of turnings, or metallic aluminum and aluminum nitride, in the form of foundry dross, in the NaOH present in excess in the carbon bottom takes place according to the equations:

IV. Combinatoin of the so obtained solutions containing sodium iluoride, sodium bicarbonate, sodium sulfate, sodium aluminate, and possibly also surplus sodium hydroxide for the purpose of precipitating the cryolite and recovering the sodium carbonate for the waste-gas washing:

The invention is further illustrated, but not limited, by the following specific examples, reference being made to the accompanying drawing in which FIGS. 1 and 2 are ow diagrams of the processes of Examples 1 and 3, respectively.

Example I lA nal lye (solution C) of the cryolite precipitation. with a pH of 11.5 containing N32C03 NaHCO3 6.6 NaF 3.3

Na2SO4 is used in wash towers 100 (PEG. 1) for the agitation of the furnace waste gases, of an aluminum electrolytic operation, flowing through these washers, whereby part of the Water used as a solvent evaporates on account of the heat content of the waste gases to be purified and is continuously replaced by further additions of final lye via a pump 101, connected to the pH control unit 102, whenever the pH of the solution circulated by pump 103 decreases to 9.0; this solution A has the following composition:

G./l. NaF(=7.1 g./l. F) 15.6 Na2CO3 12.5 Nal-11203 55.8 Na2SO4 55.5

The solution which is contaminated by the pulverulent constituents from the furnace waste gases is separated from the same in a settling basin 104 and is then led into the cryolite-recovery installation.

Concurrently with this process an amount of 4500 kg. of carbon-bottom residue which has been ground in a ball mill to a ineness of grain of 0.1 mm. maximum, and which contains 12.5% NaOH-soluble F, 4.2% NaOl-lsoluble Al and 1.2% free alkali (as NaOH), is treated with 60 rn.3 of a 1.5% caustic soda lye at 35 C. for 12 hours under constant stirring in a tank 105. The solution B resulting therefrom contains After the undissolved residue has deposited at the bottom of the reacting container, the largest part of the clear solution is decanted and thereupon the rest is separated by ltration on a revolving lter 106 from the residue. The residue containing up to 60% coke is used as fuel after drying (eg. as described with reference to FIG. 2).'

For the precipitation of the cryolite 50 m each of the solutions A and B are mixed in a reaction chamber 107 and stirred for five hours. The resulting turbid liquid with 13.8 grams cryolite per liter is separated in a Dorr thickener 108 into a slurry With 350 grams cryolite per liter and the clarified final lye. The thickened cryolite is separated from residual nal lye in a suction-cell filter 109 and is dried in a drum drier 110 at 500 to 600 C. until it contains a residual moisture of less than 1%. The cryolite consists approximately of Percent NagAlFs Nagso 5.8 Na2CO3 2.4 A1203 0.6 SiOZ 0.5 Free C 1.1 F6203 0.2 CaFZ 0.2 H2O t 0.8

The tinal lye, with a pH of 11.5 and an analysis of NaF 3.3 NaHCO3 6.6 NaZCOg and Na2SO4 is led into the gas-washing towers 100 and is again reacted with the Waste gases of the aluminum electrolyte furnace.

Example 2 In contradistinctiou to Example 1, the residue from F as NaF 8.7

Al as NaAlOZ 2.1

and

Free NaOH 7.7

To this solution are gradually added in an agitator under gas suction 630 kg. ground foundry dross containing 33% NaOH-soluble Al in order to avoid foaming over of the vessel on account of the resulting adrnixture of hydrogen and ammonia during the reaction according to the Equations 8 and 10. Stirring is continued until the evolution or gas ceases completely. After cessation of the stirring the dross residue collects at the bottom. The clear uid is then decanted and the deposit is freed from the adhering iluid on a suction filter, the fluid being then added to the decantate. The solution D obtained thereby contains F as NaF 8.7 Al as NaAlOz 5.5

and Free NaGH 2.6

For the precipitation of the cryolite 50 m of solution D are mixed with m.3 of a solution from the wastegas purification which contains NaF(=7.5 g./l. F) 16.5

NagCOg 8.7

NaHCo3 41.6

and

Ngso

The precipitated cryolite is similar in composition to that derived according to Example 1 and is separated from the final lye, filtered and dried as mentioned in that Example. The final lye with a pH of 11.0 contains NaF 4.4

NaHCO3 12.5

NazCOg and Na2SO4 35.0

and is again used as washing fluid in the gas-purification apparatus.

Example 3 During the continuous performance of the process one withdraws continuously from the carbonate washing fluid circulated by pump 203 through the several parallelworking gas-washing devices 200 (as shown in FIG. 2) 8 liters per second of a solution of a pH 9.0 containing NaF(=5.7 g./l. F) 12.6

Nal-ICOS 51.8

and

Na2SO4 58.5

connected agitators 212 to which flow simultaneously 8 liters per second of a carbon-bottom lye containing F asV NaF 11.5

Al as NaAlO2 3.5

arid

Free NaOH 1.2

The precipitation of the cryolite takes place while the mixture of the two components fiows through the five vessels 212 with a velocity of flow of 16 liters per second. The .fiuid invariably enters the vessel at the top and leaves it at the point of its cone-shaped bottom, in order to rise in a pipe line (e.g. conduits 213, shown schematically) and to iiow into the next-following vessel from the top. The turbid cryolite, iiowing from the fifth vessel 212 at the same velocity of flow as the two components are supplied, is separated from the clear final lye in a Dorr thickener 214, the iinal lye returning at 15.5 liters per second to the gas-washing towers 200 and having the following analysis:

G./l. Nar 2.6 NagCOg NaHCo3 2.9 Nagsori The cryolite thickened to a consistency of 300 grams solid mass per liter is freed from the residual final lye in a washing thickener 215.

The continuous preparation of the carbon-bottom lye used during the cryolite precipitation is accomplished as follows:

In the first of the three series-connected Dorr agitators 216 an amount of 800 grams per second of a ground carbon-bottom residue, as mentioned in Example l, containing 11.5% NaOH-soluble F, 3.5% NaOH-soluble Al and 1.2% free NaOH, is continuously added from a hopper 217 by means of a dosing screw 218. Simultaneously caustic-soda lye of 1.7% NaOH ows into the iirst vessel 216 with a velocity of iiow of 8 liters per second. The overflow of the third agitator flows into a thickener 219 in which the separation of the carbon-bottom lye from the lye residue takes place. The lye is used, as described above, for the cryolite precipitation, while from the lye residue the remainder of the lye is washed out in a centrifuge 220. The washing fiuid is used as solvent for the preparation of the caustic-soda lye.

The coke-containing residue is used as fuel after dehydration in a drying drum 221.

The described continuous cryolite-recovery process has a yield of 900 kg. cryolite per hour having the following composition:

Percent NagAlFG A1203 2.1 Na2SO4 5.2 Na2CO3 2.8 S102 0.5

F6203 CaFz 0.2 Free C 0.5 H2O 1.0

I claim:

l. A process for recovering iiuorine, as cryolite, from the ilumine-containing waste gas and carbon bottom of an aluminum electrolytic furnace, comprising the steps of treating the waste gas with a solution of sodium carbonate of a concentration suiiicient to remove at least a major portion of the iiuorine from said gas, for a period just sufficient to form an alkaline iirst solution having a pH between substantially 9 and l2 and comprising substantially equimolar quantities of sodium fluoride and sodium bicarbonate, substantially all of said sodium bicarbonate in said first solution being `derived from the reaction -between said solution of sodium carbonate and said gas; comminuting the carbon bottom, treating the comminuted carbon bottom with a solution of sodium hydroxide to form a second solution comprising sodium aluminate and sodium fluoride, and filtering said second solution to remove suspended solid particles; and mixing said first solution with said second solution to precipitate cryolite.

2. A process according to claim 1 wherein said solution of sodium hydroxide comprises a quantity of sodium hydroxide at least 30% in excess of that required to convert the tluorine and aluminum contained in said carbon bottom into sodium fiuoride and sodium aluminate, respectively.

3. A process according to claim 2, further comprising the step of adding aluminum to said second solution to produce a quantity of sodium aluminate in addition to that derived from said carbon bottom.

4. A process according to claim 1 wherein said carbon bottom is comminuted and treated with said solution of sodium hydroxide in substantially the condition in which it has been upon its removal from said furnace.

5. A process for recovering iiuorine, as cryolite, from the iiuorine-containing waste gas and the carbon bottom of an aluminum electrolytic furnace, comprising the steps of treating the waste gas with a solution of sodium carbonate of a concentration suicient to remove at least a major portion of the uorine from said gas, for a period just sufilcient to form an alkaline first solution having a pH of substantially 9 to l2 comprising substantially equimolar quantities of sodium uoride and sodium bicarbonate substantially all of said sodium bicarbonate in said first solution being derived from the reaction between said solution of sodium carbonate and said gas; cornminuting the carbon bottom, treating the comminuted carbon bottom with a solution comprising sodium hydroxide at least 30% in excess of the quantity required to convert the fiuorine and aluminum contained in said carbon bottom into sodium fluoride and sodium aluminate, respectively, to form a second solution comprising sodium aluminate and sodium iiuoride, iiltering said second solution to remove suspended solid particles, and adding aluminum particles to said second solution; and mixing said first solution with said second solution to precipitate cryolite.

6. A process for recovering fiuorine, as cryolite, from the iiuorine-containing waste gas and the carbon bottom of an aluminum electrolytic furnace, comprising the steps of treating the waste gas with a solution of sodium carbonate, of .a concentration sufiicient to remove at least a major portion of the luorine from said gas for a period just suflicient to form an alkaline first solution having a pH between substantially 9 and 12 and comprising substantially equimolar quantities of sodium fluoride .and sodium bicarbonate, substantially all of said sodium bicarbonate in said first solution being derived from the reaction between said solution of sodium carbonate and said gas; comminuting the carbon bottom, treating the comminuted carbon bottom with a solution comprising sodium hydroxide in excess of the quantity required to convert the fiuorine and aluminum contained in said carbon 'bottom into sodium fluoride and sodium aluminate, respectively, to form a second solution comprising sodium aluminate and sodium fluoride, filtering said second solution to remove suspended solid particles, and adding a sodium hydroxide-soluble aluminate to said second solution; and mixing said first solution with said second solution to precipitate cryolite.

7. A continuous process for recovering iiuorine, as cryolite, from the hydrogen fluoride-containing waste gas and the fiuorine-containing carbon bottom of an aluminum electrolytic furnace, comprising the steps of continuously treating the waste gas with a solution of sodium carbonate, of a concentration sufficient to remove at least a major portion of the iiuorine from said gas for a period sufcient to form an alka1ine iirst solution having a pH between substantially 9 and 12 and comprising substanltially equimolar quantities of sodium fluoride and sodium bicarbonate, substantially all of said sodium bicarbonate in said irst solution being derived from the reaction between said solution of sodium carbonate and said gas; treating comminuted carbon bottom with a solution of sodium hydroxide to form a second solution comprising sodium aluminate and sodium fluoride; and continuously admixing a portion of said first solution with a portion of said second solution in a proportion sufficient to precipitate substantially all the sodium aluminate from both solutions as cryolite.

8. A process for the recovery of fluorine compounds from waste gases and from the carbon bottom of an aluminum electrolytic furnace which comprises the steps of neutralizing hydrogen fluoride in the waste gases with a sodium carbonate solution having a pH between substantially 9 and 12 and of a concentration sufficient to remove at least a major portion ofthe hydrogen fluoride from said gases and to form an alkaline rst solution having a pH between substantially 9 and 12 and containing simultaneously sodium uoride and sodium bicarbonate in substantially equimolar quantities substantially all of said sodium bicarbonate in said rst solution being derived from the reaction between said solution of sodium carbonate and said gas; treating cryolite in the carbon bottom with caustic soda lye to form an aqueous second solution containing simultaneously sodium fluoride and sodium aluminate; and mixing together said first and second solutions whereby cryolite is precipitated.

9. A process for the recovery of uorine compounds from waste gases and from the carbon bottom of an aluminum electrolytic furnace which comprises the steps of neutralizing hydrogen fluoride in the waste gases with a sodium carbonate solution of a concentration suicient to remove at least a major portion of the hydrogen fluoride from said gases and to form an alkaline first solution having a pH between substantially 9 and 12 and containing simultaneously sodium uoride and sodium bicarbonate in substantially equimolar quantities, substantially all of said sodium bicarbonate in said first solution being derived from the reaction between said solution of sodium carbonate and said gas; treating cryolite in the carbon bottom with an excess of caustic soda lye to form an aqueous second solution containing simultaneously sodium iiuoride and sodium aluminate; increasing the aluminate content of said second solution by reacting aluminum with the excess caustic soda lye therein; and mixing together said first and second solutions whereby cryolite is precipitated.

References Cited in the tile of this patent UNITED STATES PATENTS 1,871,723 Morrow Aug. 16, 1932 2,597,302 Dale May 20, 1952 2,714,053 Albert et al July 26, 1955 2,813,000 Quittenton Nov. 12, 1957 FORElGN PATENTS 89,255 Norway May 20, 1957 

1. A PROCESS FOR RECOVERING FLUORINE, AS CRYOLITE, FROM THE FLUORINE-CONTAINING WASTE GAS AND CARBON BOTTOM OF AN ALUMINUM ELECTROLYTIC FURNACE, COMPRISING THE STEPS OF TREATING THE WASTE GAS WITH A SOLUTION OF SODIUM CARBONATE OF A CONCENTRATION SUFFICIENT TO REMOVE AT LEAST A MAJOR PORTION OF THE FLUORINE FROM SAID GAS, FOR A PERIOD JUST SUFFICIENT TO FORM AN ALKALINE FIRST SOLUTION HAVING A PH BETWEEN SUBSTANTIALLY 9 AND 12 AND COMPRISING SUBSTANTIALLY EQUIMOLAR QUANTITIES OF SODIUM FLUORIDE AND SODIUM BICARBONATE, SUBSTANTIALLY ALL OF SAID SODIUM BICARBONATE IN SAID FIRST SOLUTION BEING DERIVED FROM THE REACTION BETWEEN SAID SOLUTION OF SODIUM CARBONATE AND SAID GAS; COMMINUTING THE CARBON BOTTOM, TREATING THE COMMINUTED CARBON BOTTOM WITH A SOLUTION OF SODIUM HYDROXIDE TO FROM A SECOND SOLUTION COMPRISING SODIUM ALUMINATE 